This invention relates generally to vertical cavity surface-emitting lasers (VCSELs) and, more particularly, to an improved VCSEL in which the current channeling function utilizes an ion-implanted or diffused aperture region.
Semiconductor laser diodes were originally fabricated in a manner that led to a diode structure in which light is emitted parallel to the surface of the semiconductor wafer with a cavity constructed from mirrors that are perpendicular to the surface of the substrate. Unfortunately, this structure does not lend itself to low cost xe2x80x9cmassxe2x80x9d manufacturing or to the cost-effective fabrication of two-dimensional arrays of laser diodes.
These problems are overcome by a class of laser diodes that is fabricated such that the laser structure is perpendicular to the surface of the semiconductor wafer and the light is emitted perpendicular to the surface. These laser diodes are commonly known as Vertical Cavity Surface-Emitting Lasers (VCSELs). A VCSEL may be viewed as a laser having mirrors constructed from alternating layers of material having different indices of refraction. These lasers are better suited for the fabrication of arrays of lasers for displays, light sources, optical scanners, and optical fiber data links. Such lasers are useful in optical communication systems for generating the light signals carried by optical fibers and the like.
Compared to conventional edge-emitting semiconductor lasers, VCSELs have a number of desirable characteristics. The use of multi-layered DBR mirrors to form a cavity resonator perpendicular to the layers eliminates the need for the cleaving operation commonly used to create the cavity mirrors used in edge emitting lasers. The orientation of the resonator also facilitates the wafer-level testing of individual lasers and the fabrication of laser arrays.
To achieve high-speed operation and high fiber-coupling efficiency, it is necessary to confine the current flowing vertically in the VCSEL, and thus the light emission, to a small area. There are two basic prior art current confinement schemes for VCSELs. In the first scheme, a conductive aperture is defined by means of an ion-implanted, high-resistivity region in the semiconductor Distributed-Bragg-Reflector (DBR) mirror. Such a scheme is taught in Y. H. Lee, et al., Electr. Lett. Vol. 26, No. 11, pp. 710-711 (1990), which is hereby incorporated herein by reference. In this design, small ions (e.g. protons) are deeply implanted (e.g. 2.5 to 3 xcexcm) in the DBR mirror. The implantation damage converts the semiconductor material through which the ions traveled to highly resistive material. Current is provided to the light generation region via an electrode that is deposited on the top surface of the VCSEL. To facilitate a low-resistance electrical contact through which light can also exit, an annular metal contact is deposited on the top-side of the device. The contact typically has an inner diameter smaller (e.g. by 4 to 5 xcexcm) than the ion-implantation aperture in order to make contact with the non-implanted conducting area. As a result, a portion of the current-confined, light-emitting area is shadowed by the annular metal contact. The percentage of the current-confined, light-emitting area shadowed by the annular metal contact increases drastically as the size or diameter of the current-confined, light-emitting area decreases. This factor limits the smallest practical size of the current-confined area, and, therefore, limits the speed and light-output efficiency of this type of VCSEL.
In addition, ion-implanted VCSELs exhibit multiple spatial modes, which lead to a light-output-versus-current curve that often displays kinks due to the random and varying nature of the multiple spatial modes. This kinky light-output-versus-current behavior can produce a xe2x80x9cnoisyxe2x80x9d waveform, and can degrade the performance of optical communication systems based on such designs. Prior art devices have been proposed to reduce the kinks in the light-output-versus-current characteristics by introducing additional structures in the light-emitting area; however, such solutions increase the cost and complexity of the devices.
In the second design, the current confinement aperture is achieved by generating a high-resistivity oxide layer embedded in the semiconductor DBR mirror. Such a scheme is taught in D. L. Huffaker, et al., Appl. Phys. Lett., vol. 65, No. 1, pp. 97-99 (1994) and in K. D. Choquette, et al., Electr. Lett., Vol. 30, No. 24, pp.2043-2044 (1994), both of which are incorporated herein by reference. In this design, an annular insulating ring having a conducting center is generated by oxidizing one or more layers of Al-containing material in the DBR mirror in a high-temperature wet Nitrogen atmosphere. The size of the oxide aperture is determined by the Al concentration of the Al-containing layers, the temperature, the moisture concentration of the oxidizing ambient, and the length of the oxidation time. The oxidation rate is very sensitive to all of these parameters, and hence, the oxidation process is not very reproducible. As a result, device yields are less than ideal. In addition, the oxidization process leads to stress within the device, which can further reduce yields.
Broadly, it is the object of the present invention to provide an improved VCSEL and method for making the same.
The manner in which the present invention achieves its advantages can be more easily understood with reference to the following detailed description of the preferred embodiments of the invention and the accompanying drawings.
Broadly, the present invention is a current confinement element that can be used in constructing light emitting devices. The current confinement element includes a top layer and an aperture-defining layer. The top layer includes a top semiconducting material of a first conductivity type that is transparent to light. The aperture-defining layer includes an aperture region and a confinement region. The aperture region includes an aperture semiconducting material of the first conductivity type that is transparent to light. The confinement region surrounds the aperture region and includes a material that has been doped to provide a high resistance to the flow of current. In one embodiment of the present invention, the confinement region includes a confinement semiconducting material of a second conductivity type. In another embodiment of the present invention the confinement region includes a material that has been doped with impurities that increase the resistivity of the material to a value greater than 5xc3x97106 ohm-cm. The aperture-defining layer is in electrical contact with the top layer such that current will flow preferentially through the aperture region relative to the confinement region when a potential difference is applied between the top and aperture defining layers.
The present invention can be used as part of a laser diode by utilizing part of one of the mirrors as the aperture-defining layer. A laser according to the present invention is fabricated by growing the layers in the conventional manner through the light emitting layer and first spacer layer. The first few layers of the top mirror are then grown. The device is then removed from the growth chamber, and the aperture region is masked. The unmasked area is then implanted or diffused with impurities to create the confinement region. The mask is then removed, and the device is returned to the growth chamber where the remaining mirror layers are fabricated in the conventional manner.